There are three types of updates, security updates, bug fixes and new components.
(security updates) The biggest issue on a server is security, but since your server is never connected to the internet, you don't have to worry about security.
(bug fixes) If you have a version of SME that does not have a bug in the functions that you use, you should be fine. I still know people that use SME 6 without problems.
(new components) If you just use it as file server, you will not need most of the updates. You might have an issue sometime in the future when the office upgrades their computers with latest OS (windows, mac) that might require new features. But I don't think you have to worry about that for a while.
So in conclusion, if security is not an issue, than don't worry.